A study of the mechanical properties of female reproductive smooth muscle is proposed; its broad objectives are: (1) to relate the mechanical events of contraction to the underlying cellular structures and mechano-chemical proceses; (2) to develop muscle preparation lacking functional cell membranes and to study their mechanical properties when Ca-activated; and (3) to survey by means of electron microscopy the relevant ultrastructure of the mesotubarium superius muscle of rabbits, the primary tissue to be studied. To attain these objectives, I propose: (1) to measure the dynamic elastic and viscous stiffness of the muscle during isotonic and isometric contraction, and during controlled stretch and release during contraction, by recording the tension response to small sinusoidal length perturbations applied externally; (2) to treat muscles with agents known elsewhere to produce increased membrane permeability to ions, especially Ca ions and Mg ions, and to examine their mechanical properties as in (1) above; and (3) to relate the results of (1) & (2) to the overall muscle length and the hypothesized state of myofilament interaction and, considering anatomical constraints, to propose and test hypothetical mechanical schemes of the contractile process.